Facile construction of densely functionalized 4H-chromenes via three-component reactions catalyzed by l-proline

Article abstract of DOI:10.1039/c2gc35668f

Three-component reactions of salicylaldehydes, 1,3-cyclohexanediones and a sulfur, carbon, or nitrogen-based nucleophile were developed, for the first time, by using l-proline as a catalyst, which generated various substituted 4H-chromene derivatives in good to excellent yields. The reactions were performed in ethanol under mild and metal-free conditions. In these reactions, the use of l-proline as a catalyst was proven to be key for rendering the reactions possible because replacing l-proline with other acids or bases resulted in the generation of many side products. Many nucleophiles, such as thiophenols, mercaptans, indoles, sulfinic acid, benzotriazole, 2-naphthol, 5-methyl-2-phenyl-1,2-dihydropyrazol-3-one, 6-aminouracil, 6-amino-1,3-dimethyl- 1,2,3,4-tetrahydropyrimidine-2,4-dione and some commonly used C-H acids including 4-hydroxycoumarin, 2-hydroxy-1,4-naphthoquinone and barbituric acid could be successively used to assemble with dimedone and salicylaldehyde. Particularly, the product generated from the reaction of benzotriazole, 3-methoxysalicylaldehyde and dimedone can also be used as a starting material, in which the fragment of benzotriazole acts as a leaving group, for the synthesis of some important compounds by reacting with many carbon-based nucleophiles, such as dibenzoylmethane, ethyl benzoylacetate, 1,3-cyclohexanedione, N,N-dimethylaniline, 2-methylfuran and sesamol in acidic conditions. This offers a complementary route to access some 4H-chromenes that cannot be attained with the above-mentioned three-component reactions.

Full text of DOI:10.1039/c2gc35668f

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PAPER
Facile construction of densely functionalized 4H-chromenes via
three-component reactions catalyzed by L-proline†
Minghao Li,a Biao Zhanga and Yanlong Gu*a,b
Received 2nd May 2012, Accepted 13th June 2012
DOI: 10.1039/c2gc35668f
Three-component reactions of salicylaldehydes, 1,3-cyclohexanediones and a sulfur, carbon, or nitrogen-
based nucleophile were developed, for the first time, by using L-proline as a catalyst, which generated
various substituted 4H-chromene derivatives in good to excellent yields. The reactions were performed in
ethanol under mild and metal-free conditions. In these reactions, the use of L-proline as a catalyst was
proven to be key for rendering the reactions possible because replacing L-proline with other acids or bases
resulted in the generation of many side products. Many nucleophiles, such as thiophenols, mercaptans,
indoles, sulfinic acid, benzotriazole, 2-naphthol, 5-methyl-2-phenyl-1,2-dihydropyrazol-3-one,
6-aminouracil, 6-amino-1,3-dimethyl-1,2,3,4-tetrahydropyrimidine-2,4-dione and some commonly used
C–H acids including 4-hydroxycoumarin, 2-hydroxy-1,4-naphthoquinone and barbituric acid could be
successively used to assemble with dimedone and salicylaldehyde. Particularly, the product generated
from the reaction of benzotriazole, 3-methoxysalicylaldehyde and dimedone can also be used as a starting
material, in which the fragment of benzotriazole acts as a leaving group, for the synthesis of some
important compounds by reacting with many carbon-based nucleophiles, such as dibenzoylmethane,
ethyl benzoylacetate, 1,3-cyclohexanedione, N,N-dimethylaniline, 2-methylfuran and sesamol in acidic
conditions. This offers a complementary route to access some 4H-chromenes that cannot be attained with
the above-mentioned three-component reactions.
pharmacological properties such as spasmolytic, diuretic, anti-
Introduction
coagulant, anticancer, and antianaphylactic activities.4 For this
reason, the synthesis of 4H-chromene derivatives has recently
attracted great interest, and until now various methods have been
developed for synthesizing 4H-chromenes, such as (i) the cyclo-
addition of phenols with ketones or propargylic alcohols, which
generally uses gold or ruthenium complexes as catalysts;5
(ii) DABCO-catalyzed reactions of salicyl N-tosylimines with
allenic esters;6 (iii) the acid-catalyzed tandem benzylation and
cyclization of 1,3-dicarbonyl compounds using benzylic alcohols
as alkylating agents;7 (iv) copper-mediated aryl vinyl ether
formation followed by a ruthenium-mediated RCM reaction;8
(v) CuI-catalyzed intramolecular or intermolecular coupling of
aryl bromides with 1,3-dicarbonyls;9 (vi) a tetrahydrothiophene-
catalyzed ylide annulation reaction, via a tandem Michael
addition/elimination/substitution reaction.10 Recently, a method
for preparing chiral 4H-chromenes with a α-hydroxy carboxylate
moiety has also been developed via an iminium-allenamine
engaged oxa-Michael-aldol cascade reaction.11
Although various 4H-chromenes have been successfully syn-
thesized by these methods (see ESI, Scheme S1†), all of these
approaches suffer from either a limited diversity of available
starting materials or the use of expensive and toxic catalysts.
In most cases, the generation of a large amount of waste is inevi-
table. When a complex substrate that is not commercially avail-
able is used, the E-factor of synthesizing 4H-chromenes will be
further increased. Furthermore, volatile and toxic solvents are
Maximizing synthetic efficiency by designing a complexity-
generating diversity-oriented synthesis is gaining more and more
importance in organic synthesis and drug discovery. For this
purpose, development and utilization of multi-component reac-
tions (MCRs) are of prime importance.1 Generally, a product in
a MCR is assembled according to a cascade of elementary
chemical reactions. Thus, there is a network of reaction equili-
bria, in which all the starting materials finally flow into an ir-
reversible step yielding the product. The challenge is thus to
conduct a MCR in such a way that the network of pre-
equilibrated reactions channels into the main product and does
not yield side products.2
Heterocycles such as chromans and chromenes are important
classes of benzopyran derivatives to be found in many natural
products.3 Particularly, 4H-chromene derivatives show various
aInstitute of Physical Chemistry and Industrial Catalysis, School of
Chemistry and Chemical Engineering, Hubei Key Laboratory of
Material Chemistry and Service Failure, Huazhong University of
Science and Technology (HUST), 1037 Luoyu Road, Hongshan District,
Wuhan 430074, China. E-mail: klgyl@hust.edu.cn;
Fax: +(0)86-(0)27-87 54 45 32
bState Key Laboratory for Oxo Synthesis and Selective Oxidation,
Lanzhou Institute of Chemical Physics, Lanzhou, 730000, P.R. China
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c2gc35668f
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Table 1 Three-component reaction of salicyladehyde (1a), dimedone
(2a) and thiophenol (3a) under different conditionsa
also used in these reported methods. In order to circumvent these
problems, a general, efficient, and environmentally benign
method for synthesizing 4H-chromenes is required.
The synthesis of many important heterocycles could be
realized by MCRs. One of the advantages of synthesizing hetero-
cycles via MCRs is the great ability of creating molecular com-
plexicity and diversity. However, MCRs that are capable of
providing 4H-chromenes are rather limited. For example,
three-component condensation reactions of aromatic aldehydes,
1,3-cyclohexanediones and phenols were known to be able to
generate 4H-chromenes. Unfortunately, this method only works
with highly reactive phenols, such as sesamol, and therefore
cannot be widely used in the synthesis of 4H-chromenes.12
Perumal et al.13 have described a one-pot, three-component reac-
tion of salicylaldehyde, malononitrile and a nucleophile, which,
however, only generated a special class of 4H-chromenes,
2-amino-4H-chromenes.14
In order to develop a general, practicable and an environmen-
tally benign method for the synthesis of 4H-chromenes, we
started, some time ago, a research program on this topic. During
our investigation, we viewed that it might be possible to prepare
4H-chromenes through a one-pot three-component reaction of
salicylaldehyde, cyclo-1,3-dicarbonyl compound and a nucleo-
phile. However, in view of the fact that cyclo-1,3-dicarbonyl
compounds and nucleophiles are both reactive in the electro-
philic alkylation of salicylaldehyde,15 in order to get the desired
three-component adduct, extra effort should be paid to control
the reaction selectivity. Herein, we disclose the successful
outcome of this endeavor in which a new class of densely substi-
tuted 4H-chromenes was prepared in good to excellent yields via
a three-component reaction of salicylaldehyde, 1,3-cyclohexane-
dione and a suitable nucleophile. In our system, the use of
L-proline as catalyst is the key for rendering the three-component
reactions possible. All the reactions were performed in ethanol
under mild and metal-free conditions. In most cases, the products
were obtained with exclusive selectivity, and therefore, the only
by-product generated in this system is water. All these properties
endowed our system with a salient feature on meeting the
requirement of current innovation toward green organic
synthesis.
Yield (%)
Entry Catalyst
Solvent
4a
4a–a 4a–b
1
2
—
TsOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
—
—
—
—
—
—
CH3CN
DCE
Trace 33
30
Trace 20
Trace
5
19
7
3
4
Sc(OTf)3
FeCl3
5
Trace 10
5
6
H3BO3
I2
9
40% Trace
Trace Trace 40
7
8
NaOH
K2CO3
0
0
0
46
45
42
35
0
15
12
18
17
15
18
6
28
30
33
20
0
0
Trace
35
0
Trace
19
20
18
19
22
Trace
Trace
Trace
Trace
8
Trace
Trace
9
Piperidine
DABCO
L-Proline
N-Methyl-L-proline
L-Proline methyl ester
L-Pipecolic acid
Sarcosine
3-Piperidinecarboxylic acid
L-Proline
L-Proline
L-Proline
L-Proline
L-Proline
10
11
12b
13b
14b
15b
16b
17
18
19
20
21
22
23c
24d
25e
12
80
46
45
49
50
46
61
75
37
1,4-Dioxane 35
Toluene 18
[BMIm]BF4 52
L-Proline
L-Proline
L-Proline
L-Proline
EtOH
EtOH
EtOH
88
63
87
Trace Trace
Trace Trace
a 1a: 0.5 mmol, 2a: 0.5 mmol, 3a: 0.5 mol, solvent: 1.2 ml, catalyst:
0.05 mmol, 80 °C, 1.5 h. b Solvent: ethanol. c Reaction time: 3 h.
d Temperature, 50 °C, 3 h. e Reaction scale: 10.0 mmol, 3 h.
obtained 4H-chromene skeleton, which seriously limits its poten-
tial capacity for creating molecular diversity. Therefore, an
efficient method to construct 4a-type 4H-chromenes is strongly
desired. Such a one-pot three-component synthesis of 4H-chro-
menes, as shown in Table 1, if established, is a huge incentive.
In Table 1, the reaction was performed in ethanol at 80 °C.
Under catalyst-free conditions, although salicylaldehyde was
almost consumed after 1.5 h, only a trace amount of 4a was
formed (entry 1). Two by-products, 4a–a and 4a–b, could be
isolated from the reaction mixture. As the model three-
component reaction seems to involve the electrophilic alkylation
of aldehyde, which generally requires the assistance of an acid
catalyst, we therefore examined some commonly used acidic
catalysts. However, with strong acids, such as TsOH, Sc(OTf)3
and FeCl3, the yield of 4a did not significantly improve. In these
cases, an etherification of dimedone with ethanol that generates
3-ethoxy-5,5-dimethyl-2-cyclohexen-1-one was clearly observed
(Table 1, entries 2 to 4).17 This etherification can be significantly
suppressed by using weak acids, such as H3BO3 and I2, as
Results and discussion
Initially, a three-component reaction of salicylaldehyde (1a),
dimedone (2a) and thiophenol (3a) was used as a model reaction
in order to find an appropriate catalyst (Table 1). We expected
that the carbonyl group of salicylaldehyde could react simulta-
neously with both 2a and 3a to form an intermediate that then
underwent an intramolecular dehydration to generate a densely
substituted 4H-chromene 4a. A literature survey suggested there
is only one report on the synthesis of 4a-type of 4H-chromene
(or 2,3,4,9-tetrahydro-xanthen-1-one).16 However, the reported
method suffers from two drawbacks: (i) a tedious and complex
operation which involves a base-catalyzed reductive alkylation
reaction of salicylaldehyde and dimedone using a Hantzsch ester
as a reducing reagent and then, an acid-catalyzed intramolecular
cyclization of the generated product in the first step, and (ii)
failure to introduce a substituent on the C9 position of the
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catalysts. However, the formation of by-products is still predomi-
nant in these cases (Table 1, entries 5 and 6). We then examined
some basic catalysts. It was found that all the examined bases
including NaOH, K2CO3, piperidine and DABCO were ineffec-
tive for generating the desired product, 4a (Table 1, entries 7
to 10). It should be noted that the product distribution depended
very much on the catalyst used in the model reaction. For
example, when I2 was used as catalyst, formation of 4a–b was
predominant, whereas only 4a–a was detected using inorganic
bases, such as NaOH and K2CO3, as catalyst. These results
imply that the properties of the catalyst have a remarkable effect
in controlling the reaction selectivity. This gave us an impetus to
further explore the feasibility of constructing 4a from 1a, 2a and
3a by means of changing the catalyst.
L-Proline has been widely used as a catalyst in organic reac-
tions.18 Particularly, Kumar et al.19 have recently described
an impressive example, in which L-proline was proved to be
a unique catalyst for the promotion of a three-component
Mannich-type reaction of secondary amines, aldehyde and
indoles that generates 3-amino-alkylated indoles in high yields.
The key to Kumar’s system comes from a unique ability of
L-proline to suppress an intrinsic affinity of indole to aldehyde,
which otherwise hampers the formation of the three-component
Mannich product. Inspired by Kumar’s report, we envisaged that
L-proline might be a good catalyst for our model three-
component reaction because the random reactions of 1a with two
nucleophiles, 2a and 3a, require an unambiguous compounding
in order to obtain 4a. To our great delight, by using L-proline as
catalyst, formation of by-products, 4a–a and 4a–b, was totally
suppressed in ethanol, and as a result, the yield of 4a was signifi-
cantly improved (Table 1, entry 11). The unique catalytic activity
of L-proline may stem from its amphoteric nature that inherently
results from the presence of both amine and carboxylic acid
functionalities. The amino group might result in the formation of
imine and the carboxylic group could stabilize the charge gene-
rated on the nitrogen.20 When N-methyl proline was used in the
reaction, the desired product was obtained in 46% of yield, like-
wise proline methyl ester was found to be less efficient for cata-
lyzing the reaction (Table 1, entries 12 and 13). Other amine
acids including L-pipecolic acid, sarcosine and 3-piperidine-
carboxylic acid were also investigated, and the yields obtained
were far from satisfactory (Table 1, entries 14 to 16). The results
showed that L-proline was the best organocatalyst for the three-
component reaction of salicylaldehyde, dimedone and sulfide.
Further investigation revealed that the performance of L-proline
catalyst was strongly affected by the reaction solvent. While 4a
was obtained in 80% of yield in ethanol, the reaction under
solvent-free conditions gave only 61% of yield (Table 1, entry
17). Acetonitrile also works well as a solvent, but the yield of 4a
was slightly inferior as compared with that of ethanol (Table 1,
entry 18). Due to an extensive formation of 4a–a, the yields
obtained in DCE, 1,4-dioxane, toluene and an ionic liquid,
[BMIm]BF4, are rather poor (Table 1, entries 19 to 22). In
ethanol, the yield of 4a could be further increased to 88% by
increasing the reaction time to 3 h (Table 1, entry 18). However,
a decrease in reaction temperature from 80 to 50 °C resulted in
an appreciable loss of the reaction yield (Table 1, entry 19).
Therefore, the optimal condition for L-proline catalyst system is
ethanol solvent, 80 °C, 3 h. It should be noted that, under the
Fig. 1 Substrate scope of the three-component reaction of salicylalde-
hyde, 1,3-cyclohexanedione and sulphide.
optimal condition, a reaction scaled up to 10 mmol provided
uniform results (Table 1, entry 20), indicating the usefulness of
the present three-component reaction for practical synthesis.
With the optimized conditions in hand, we probed the scope
of the reaction. As evidenced by the results in Fig. 1, thiophenols
with different substituents, such as methyl, methoxyl, isopropyl,
all smoothly reacted, producing the corresponding 4H-chro-
menes in generally good to excellent yields (4a–4g). Mercaptans
were proved to be more competent than thiophenols (4h–4j).
Substituents on salicylaldehyde, such as methoxyl, ethoxyl and
bromo, had no significant influence on the performance (4k–4m)
of the three-component reaction. However, 1,3-cyclohexanedione
was not as competent as the substituted one (4a, 4n–4p).
In order to rationalize the formation of 4a in the L-proline/
ethanol system, we then investigated the possibility of synthe-
sizing 4a from 4a–a or 4a–b under standard conditions, without
success (ESI, Scheme S2†). On the basis of this fact, a plausible
mechanism was then proposed, as shown in Fig. 2. The reaction
commenced with the formation of an imine intermediate 5a from
salicylaldehyde and L-proline.21 Then 5a underwent a nucleophi-
lic addition of thiophenol (Fig. 2, pathway a) or dimedone
(Fig. 2, pathway b) to form the corresponding Mannich-type
intermediate 5b or 5c, which is susceptible towards nucleophilic
attack of the other nucleophile, providing the desired product
after an intramolecular dehydration. Although pathway a and
pathway b are both able to generate 4a, we believe that the reac-
tion might proceed predominantly via pathway b. The reasons
are listed below: (i) because 5b and 5c contain a COO− group in
close proximity to a hydroxy functionality, it is not unreasonable
to expect that an intramolecular hydrogen bond will be formed
in the intermediates 5b and 5c. However, in the case of 5c, there
are two hydroxy groups able to interact with the oxygen of the
COO− fragment. As the proton in the enolic hydroxy group of
the dimedone fragment should be more active than that of the
phenolic hydroxy group,22 COO− in 5c should tend to interact
with the enolic proton; this kind of intramolecular interaction
might be stronger than that of 5b, thus ensuring a better stability
of 5c. (ii) The high selectivity observed in the L-proline system
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Fig. 2 The proposed mechanism of the multi-component reaction cata-
lyzed by L-proline.
Scheme 1 Three-component reaction of salicylaldehydes, cyclo-1,3-
hexanediones and indoles catalyzed by L-proline.
can be explained by adopting 5c as the intermediate. The pheno-
lic hydroxy group in 5c can also interact with the keto oxygen.
The presence of these two kinds of intermolecular hydrogen
bond significantly decrease the flexibility of 5c. As a result, the
next nucleophile has to approach the active site of 5c at a fixed
angle. Considering the large molecular size and the heavy steric
hindrance of 5c, a needle-type nucleophile should be a perfect
choice for converting 5c; therefore, thiophenol has the priority as
compared with dimedone to act as a second nucleophile, which
ensures the high selectivity of the model reaction. On the con-
trary, because intermediate 5b is flexible, both thiophenol and
dimedone can interact with 5b, thus, it is unreasonable to expect
that a high selectivity could be obtained if 5b was adopted as the
reaction intermediate.
Indoles are an important class of carbon-based nucleophiles.
In view of the fact that an indole fragment is found in many bio-
logically active compounds,23 we therefore investigated the feasi-
Fig. 3 Progress of reaction: (a) beginning of reaction; (b) reaction as it
progresses; (c) end of reaction.
bility of using indole as
a nucleophile to react with
salicylaldehyde and dimedone. As shown in Scheme 1, the
multi-component reaction of indole proceeded very well, and
importantly, it displayed a very good substrate compatibility.
Indoles, salicylaldehydes or cyclo-1,3-hexanediones containing
various substituents could all be successfully used in this reac-
tion, producing the corresponding products in good to excellent
yields. It should be noted that the products in the reactions of
indole could be easily isolated without the use of silica column
chromatography. As showed in Fig. 3, all reactants dissolved
well in ethanol in the beginning of the reaction. As the reaction
progressed, some insoluble species were gradually precipitated
out, and at the end of the reaction, a large amount of solid accu-
mulated in the bottom of the vessel. As a result of this behavior,
all the products (6a–6p) could be obtained by filtration. This
procedure imbues the synthetic methodology with green
credentials.
With these results in hand, we then examined the possibility
of synthesising other 4H-chromenes via this method. Many other
nucleophiles, such as 5-methyl-2-phenyl-1,2-dihydropyrazol-
3-one, 6-amino-1,3-dimethylpyrimidine-2,4(1H,3H)-dione, 6-amino-
pyrimidine-2,4(1H,3H)-dione, 6-bromonaphthalen-2-ol, 2-hydro-
xynaphthalene-1,4-dione, barbituric acid, 4-hydroxy-2H-
chromen-2-one and benzenesulfinic acid could also be success-
fully employed to react with salicylaldehyde and dimedone,
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Table 2 Three component reaction of salicylaldehyde, dimedone and
other carbon nucleophilesa
Entry
1
NuH
Time (h)
8
Product
Yieldb (%)
Scheme 2 Multi-component reactions with benzotriazole.
85 (7a)
affording 9-substituted 4H-chromenes in moderate to excellent
yields. A nitrogen-based nucleophile, benzotriazole, could also
be used without affecting the reaction yield (Scheme 2). It is
noteworthy that in the reactions of Table 2 and Scheme 2, the
products were all obtained by filtration, manifesting clearly the
green aspect of this method. Previously, benzotriazole has been
used as a leaving group in organic synthesis.24 Encouraged by
this fact, we then investigated the possibility of using 7i or 7j as
the starting material for this organic synthesis. As shown in
Scheme 3, under acidic conditions, the product 7i reacted readily
with many carbon-based nucleophiles including dibenzoyl-
methane, ethyl benzoylacetate, 1,3-cyclohexanedione, N,N-
dimethylaniline and 2-methylfuran, affording the corresponding
densely functionalized 4H-chromenes in moderate to excellent
yields. Importantly, the skeleton of 8c was known to possess bio-
logical activity.25 Therefore, the method in Scheme 3 not only
expands the synthetic capacity of the three-component reaction,
but also opens an effective way for accessing some valuable
compounds.
2c
11
11
63 (7b)
92 (7c)
3c
4
5
8
6
52 (7d)
99 (7e)
Conclusions
We have developed a general, practical, and environmentally
benign method to construct densely functionalized 4H-chro-
menes via the three-component reaction of salicylaldehydes and
1,3-cyclohexanediones by using L-proline as catalyst. All the
reactions were performed in ethanol under mild and metal-free
conditions. We suggest that the reaction may proceed via an
enolic hydroxy-assisted mechanism. Many carbon, sulfur, and
nitrogen-based nucleophiles could be successfully used to react
with salicylaldehyde and 1,3-cyclohexanediones. Particularly,
some products could be easily isolated by filtration, avoiding the
use of silica column chromatography. In addition, the product
generated from the reaction of benzotriazole, 3-methoxysalicyl-
aldehyde and dimedone could also be used as a starting material,
in which the fragment of benzotriazole acted as a leaving group,
to react with many carbon-based nucleophiles under acidic con-
ditions. This offers an alternative method for the synthesis of
4H-chromenes that cannot be attained with the above-mentioned
three-component reaction.
6
7
8
6
6
8
87 (7f)
90 (7g)
92 (7h)
a Salicylaldehyde: 0.5 mmol, dimedone: 0.5 mmol, carbon nucleophile:
0.5 mmol, L-proline: 0.05 mmol, 80 °C. b Yield obtained by filtration.
c Yield obtained by recrystallization in DMSO–EtOH (1 : 4).
Experimental
General
Melting points were determined by a microscopic melting point
meter and were uncorrected. IR spectra were recorded on a
FT-IR Bruker (VERTEX 70) spectrometer using KBr pellets or
in ppm relative to TMS in solvent. All chemicals used were of
reagent grade and were used as received without further purifi-
cation. All reactions were conducted in a 10 mL V-type flask
equipped with triangle magnetic stirring.
1
neat liquid technology. H and 13C NMR spectra were recorded
on a Bruker AV-400 spectrometer. Chemical shifts are expressed
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Scheme 3 Substitution reaction of 7i with various carbon-based nucleophiles.
General procedure of three-component reaction of salicylalde-
hyde 1,3-cyclohexanedione and thiophenol. A mixture of salicyl-
aldehyde (0.5 mmol), 1,3-cyclohexanedione (0.5 mmol),
thiophenol (0.5 mmol) and L-proline (0.05 mmol) in ethanol
(1.2 mL) was stirred at 80 °C for 3 h. After reaction completion,
the solution was subjected to isolation with preparative TLC
using a mixture of ethyl acetate and petroleum ether (1 : 5) as
eluent. The scale up experiment was taken under similar pro-
cedure. After reaction, ethanol was evaporated under reduced
pressure. The residual liquid was subjected to isolation with
silica gel column chromatography.
General procedure of the substitution reaction of 7i. A
mixture of 7i (0.3 mmol) and nucleophile (1.1 equiv.) in corres-
ponding conditions. After reaction, the product was purified by a
preparative TLC plate using a mixture of ethyl acetate and
petroleum ether as eluent.
Partial spectroscopic data of the obtained new compounds
3,3-Dimethyl-9-(phenylthio)-2,3,4,9-tetrahydro-1H-xanthen-
1
1-one (4a). Brown liquid; H NMR (CDCl3): 0.97(s, 3H), 1.06
(s, 3H), 2.15–2.32 (m, 4H), 5.21 (s, 1H), 6.70 (dd, Ja = 0.8 Hz,
Jb = 8.0 Hz, 1H), 6.96 (dd, Ja = 0.8 Hz, Jb = 8.0 Hz, 2H),
6.99–7.09 (m, 4H), 7.13–7.20 (m, 2H); 13C NMR: 28.4, 28.5,
32.1, 40.8, 41.3, 50.9, 109.8, 116.1, 122.7, 125.0, 128.1, 128.2,
128.7, 129.5, 131.8, 136.4, 150.6, 166.3, 195.8; IR (cm−1):
3046, 2959, 2935, 2869, 1661, 1638, 1578, 1480, 1458, 1437,
1382, 1232, 1176, 1139, 1098, 1021, 801, 757, 746, 692;
HRMS m/z (ESI) calcd for C21H20NaO2S [M + Na]+ 359.1082
found 359.1080.
General procedure of three-component reaction of salicylalde-
hyde 1,3-cyclohexanedione and other nucleophile. A mixture of
salicylaldehyde (0.5 mmol), 1,3-cyclohexanedione (0.5 mmol),
nucleophile (0.5 mmol) and L-proline (0.05 mmol) in ethanol
(1.2 mL) was stirred at 80 °C for several hours (as shown
above). After reaction, the mixture was cooled nearly to zero
degree. Then it was filtered and washed with ethanol to afford an
analytically pure product except for 7b and 7c which needed to
recrystallize in DMSO–EtOH (1 : 4).
5-Methoxy-3,3-dimethyl-9-(2-phenyl-1H-indol-3-yl)-2,3,4,9-tetra-
hydro-1H-xanthen-1-one (6a). White solid, mp = 233–235 °C;
Green Chem.
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1H NMR (DMSO-d6): 0.95 (s, 3H), 1.08 (s, 3H), 2.19 (d, J =
16.4 Hz, 1H), 2.30 (d, J = 16.4 Hz, 1H), 2.63–2.65 (m, 2H),
3.80 (s, 3H), 5.41(s, 1H), 6.20 (d, J = 5.6 Hz, 1H), 6.79 (d, J =
6.8 Hz, 2H), 6.88 (d, J = 7.6 Hz, 1H), 6.99 (d, J = 7.6 Hz, 1H),
7.28–7.32 (m, 2H), 7.46 (t, J = 7.6 Hz, 1H), 7.59 (d, J = 7.6 Hz,
2H), 8.13 (d, J = 6.4 Hz, 2H), 11.20 (s, 1H); 13C NMR: 27.3,
28.6, 29.1, 32.0, 41.1, 50.9, 56.1, 110.3, 111.3, 111.8, 116.1,
118.7, 119.1, 120.8, 121.5, 124.8, 126.3, 126.8, 128.1, 128.3,
128.5, 129.1, 129.6, 133.7, 134.6, 136.4, 138.2, 147.5, 164.9,
197.4; IR (cm−1): 3441, 3411, 3325, 3056, 2953, 2932, 2873,
1636, 1612, 1584, 1481, 1456, 1374, 1273, 1222, 1187, 1115,
1091, 928, 765, 738, 700; HRMS m/z (ESI) calcd for
C30H27NNaO3 [M + Na]+ 472.1889 found 472.1885.
13C NMR: 13.6, 27.1, 29.4, 31.3, 32.2, 41.6, 50.8, 56.0, 105.6,
106.2, 110.0, 110.2, 121.1, 124.0, 124.5, 139.1, 147.7, 150.7,
154.8, 165.4, 196.7; IR (cm−1): 3107, 3070, 2966, 2942, 1640,
1610, 1582, 1484, 1464, 1384, 1273, 1232, 1187, 1123, 1092,
1015, 952, 788, 759, 737, 564; HRMS m/z (ESI) calcd for
C21H22NaO4 [M + Na]+ 361.1416 found 361.1410.
Ethyl 2-(5-methoxy-3,3-dimethyl-1-oxo-2,3,4,9-tetrahydro-1H-
xanthen-9-yl)-3-oxo-3-phenylpropanoate (8c). White solid, mp
= 180–181 °C; 1H NMR (CDCl3): 0.99 (t, J = 9.6 Hz, 3H), 1.14
(s, 6H), 2.32 (d, J = 16.0 Hz, 1H), 2.37 (d, J = 16.0 Hz, 1H),
2.59 (s, 2H), 3.84 (s, 3H), 3.88–3.96 (m, 2H), 4.76 (d, J = 2.4
Hz, 1H), 4.86 (d, J = 3.2 Hz, 1H), 6.62 (d, J = 7.6 Hz, 1H), 6.78
(d, J = 8.8 Hz, 1H), 6.91 (d, J = 8.0 Hz, 1H), 6.94 (d, J =
8.0 Hz, 1H), 7.49–7.61 (m, 3H), 8.22 (d, J = 7.6 Hz, 2H);
13C NMR: 13.8, 26.7, 29.9, 32.2, 41.8, 50.9, 56.1, 59.4, 61.0,
110.4, 111.0, 121.4, 122.1, 124.2, 128.2, 128.6, 128.9, 129.0,
133.6, 136.2, 147.6, 168.0, 168.7, 194.7, 197.7; IR (cm−1):
3043, 2990, 2963, 2944, 1749, 1677, 1630, 1582, 1484, 1448,
1389, 1269, 1236, 1197, 1164, 1129, 1095, 1018, 773, 730, 689,
645; HRMS m/z (ESI) calcd for C27H28NaO6 [M + Na]+
471.1784 found 471.1778.
9-(5-Hydroxy-3-methyl-1-phenyl-1H-pyrazol-4-yl)-3,3-dimethyl-
2,3,4,9-tetrahydro-1H-xanthen-1-one (7a). White solid, mp =
1
207–209 °C; H NMR (DMSO-d6): 1.02 (s, 3H), 1.06 (s, 3H),
2.13–2.69 (m, 4H), 2.50–2.55 (m, 3H), 4.86 (br, 1H), 7.06–7.19
(m, 5H), 7.37 (t, J = 4.0 Hz, 2H), 7.65 (d, J = 4.0 Hz, 2H), 10.6
(s, 1H); 13C NMR: 28.9, 29.4, 29.7, 30.8, 32.5, 40.6, 50.3,
111.4, 117.6, 120.0, 127.6, 129.3, 133.4, 145.0, 150.6, 152.0,
1524.3, 155.3, 161.8, 162.5, 195.8; IR (cm−1): 3430, 3063,
2957, 2940, 2816, 1632, 1579, 1497, 1382, 1234, 1180, 757,
685, 663; HRMS m/z (ESI) calcd for C25H24N2NaO3 [M + Na]+
423.1685 found 423.1680.
2-(5-Methoxy-3,3-dimethyl-1-oxo-2,3,4,9-tetrahydro-1H-xanthen-
9-yl)-1,3-diphenylpropane-1,3-dione (8d). Red solid, mp
=
1
177–178 °C; H NMR (CDCl3): 0.92 (s, 3H), 1.09 (s, 3H), 2.24
(d, J = 16.0 Hz, 1H), 2.35 (d, J = 16.0 Hz, 1H), 2.53 (d, J =
17.6 Hz, 1H), 2.57 (d, J = 17.6 Hz, 1H), 3.89 (s, 3H), 4.84 (d,
J = 3.6 Hz, 1H), 6.11 (d, J = 3.6 Hz, 1H), 6.42 (dd, Ja = 0.8 Hz,
Jb = 7.6 Hz, 1H), 6.80 (dd, Ja = 1.2 Hz, Jb = 8.0 Hz, 1H),
6.91 (t, J = 8.0 Hz, 1H), 7.32 (t, J = 8.0 Hz, 2H), 7.45 (t, J =
7.6 Hz, 1H), 7.59–7.72 (m, 5H), 8.45 (d, J = 6.8 Hz, 2H);
13C NMR: 26.9, 29.1, 31.9, 33.0, 41.5, 50.7, 55.9, 58.9, 110.0,
110.7, 120.8, 122.2, 123.7, 127.8, 128.4, 129.0, 129.1, 132.7,
133.7, 135.5, 137.0, 141.0, 147.2, 168.5, 195.4, 195.6, 198.0;
IR (cm−1): 3058, 2960, 2940, 1686, 1644, 1584, 1486, 1444,
1389, 1267, 1232, 1207, 1093, 1005, 776, 743, 690, 647;
HRMS m/z (ESI) calcd for C31H28NaO5 [M + Na]+ 503.1834
found 503.1830.
9-(1H-Benzo[d][1,2,3]triazol-1-yl)-5-methoxy-3,3-dimethyl-
2,3,4,9-tetrahydro-1H-xanthen-1-one (7i). White solid, mp =
182–184 °C; H NMR (DMSO-d6): 0.98 (s, 3H), 1.10 (s, 3H),
1
2.23 (d, J = 16.4 Hz, 1H), 2.37 (d, J = 16.4 Hz, 1H), 2.78 (d,
J = 17.6 Hz, 1H), 2.85 (d, J = 17.6 Hz, 1H), 3.89 (s, 3H), 6.82
(dd, Ja = 2.0 Hz, Jb = 7.2 Hz, 1H), 7.08 (d, J = 2.8 Hz, 1H),
7.23 (s, 1H), 7.36–7.40 (m, 1H), 7.59 (dt, Ja = 0.2 Hz, Jb =
8.0 Hz, 1H), 7.97 (d, J = 8.4 Hz, 1H), 8.06 (d, J = 8.4 Hz, 1H);
13C NMR: 26.3, 28.4, 31.9, 40.4, 47.7, 49.7, 55.8, 107.6, 110.6,
112.2, 119.0, 119.8, 120.4, 123.9, 125.3, 127.3, 131.9, 139.0,
144.8, 147.5, 167.6, 195.7; IR (cm−1): 3092, 3058, 2961, 2937,
2891, 2840, 1651, 1614, 1586, 1483, 1448, 1384, 1268, 1234,
1195, 1132, 1087, 756, 747; HRMS m/z (ESI) calcd for
C22H21N3NaO3 [M + Na]+ 398.1481 found 398.1475.
9-(6-Hydroxybenzo[d][1,3]dioxol-5-yl)-5-methoxy-3,3-dimethyl-
2,3,4,9-tetrahydro-1H-xanthen-1-one (8e). White solid, mp =
204–205 °C; 1H NMR (CDCl3): 1.01 (s, 3H), 1.11 (s, 3H),
2.24 (d, J = 16.4 Hz, 1H), 2.31 (d, J = 16.4 Hz, 1H), 2.51 (s,
2H), 3.85 (s, 3H), 5.21 (s, 1H), 5.86 (d, J = 1.6 Hz, 1H), 5.90
(d, J = 1.6 Hz, 1H), 6.43 (q, J = 1.6 Hz, 1H), 6.56 (d, J =
5.6 Hz, 2H), 6.64–6.72 (m, 2H), 7.58 (s, 1H); 13C NMR: 27.6,
29.5, 31.5, 32.4, 41.9, 50.8, 56.2, 98.1, 101.7, 108.7, 109.5,
112.6, 117.6, 129.4, 121.3, 133.1, 143.0, 144.5, 145.1, 147.1,
148.5, 166.4, 198.8; IR (cm−1): 3422, 3359, 3005, 2953, 2879,
1655, 1619, 1481, 1379, 1281, 1218, 1144, 1033, 930, 882, 730,
550; HRMS m/z (ESI) calcd for C23H22NaO6 [M + Na]+
417.1314 found 417.1310.
9-(2-Hydroxy-6-oxocyclohex-1-en-1-yl)-5-methoxy-3,3-dimethyl-
2,3,4,9-tetrahydro-1H-xanthen-1-one (8a). White solid, mp =
1
180–181 °C; H NMR (DMSO-d6): 1.78 (s, 3H), 1.83 (s, 3H),
2.65–2.37 (m, 10H), 4.57 (s, 3H), 5.86 (s, 1H), 7.35 (d, J =
7.6 Hz, 1H), 7.58–7.60 (m, 1H), 7.70 (t, J = 8.0 Hz, 1H),
11.32 (s, 1H); 13C NMR: 20.4, 26.3, 27.6, 29.0, 31.5, 36.6, 40.7,
50.3, 55.4, 109.5, 109.6, 119.7, 123.8, 126.2, 139.1, 146.6,
164.6, 195.7; IR (cm−1): 3231, 3020, 2945, 2893, 2872, 1644,
1587, 1479, 1376, 1274, 1228, 1194, 1090, 981, 747; HRMS
m/z (ESI) calcd for C22H24NaO5 [M + Na]+ 391.1521 found
391.1515.
5-Methoxy-3,3-dimethyl-9-(5-methylfuran-2-yl)-2,3,4,9-tetra-
hydro-1H-xanthen-1-one (8b). Red solid, mp = 136–137 °C;
1H NMR (CDCl3): 1.12 (s, 6H), 2.13 (s, 3H), 2.30 (s, 2H),
2.59 (d, J = 17.6 Hz, 1H), 2.65 (d, J = 17.6 Hz, 1H), 3.87 (s,
3H), 5.06 (s, 1H), 5.75 (dd, Ja = 1.2 Hz, Jb = 3.2 Hz, 1H), 5.87
(d, J = 3.2 Hz, 1H), 6.77 (dd, Ja = 1.2 Hz, Jb = 8.0 Hz, 1H),
6.87 (dd, Ja = 0.8 Hz, Jb = 8.0 Hz, 1H), 7.00 (t, J = 8.0 Hz, 1H);
9-(4-(Dimethylamino)phenyl)-5-methoxy-3,3-dimethyl-2,3,4,9-
tetrahydro-1H-xanthen-1-one (8f). White solid, mp
=
179–180 °C; 1H NMR (CDCl3): 1.02 (s, 3H), 1.09 (s, 3H),
2.19 (d, J = 16.4 Hz, 1H), 2.25 (d, J = 16.4 Hz, 1H), 2.57 (d,
J = 17.6 Hz, 1H), 2.64 (d, J = 17.6 Hz, 1H), 2.83 (s, 6H),
3.88 (s, 3H), 4.90 (s, 1H), 6.57 (d, J = 8.8 Hz, 2H), 6.70 (t, J =
This journal is © The Royal Society of Chemistry 2012
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112.8, 113.9, 121.7, 124.8, 127.3, 128.6, 134.6, 139.2, 147.9,
149.3, 164.0, 197.1; IR (cm−1): 3089, 2957, 2889, 2839, 1647,
1611, 1583, 1518, 1481, 1375, 1272, 1225, 1189, 1117, 1089,
950, 912, 837, 785, 730, 647, 550; HRMS m/z (ESI) calcd for
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Acknowledgements
The authors thank the National Natural Science Foundation of
China for their financial support (21173089). The authors are
also grateful to all staff members in the Analytical and Testing
Center of HUST, for their contributions to our works. The
Program For New Century Excellent Talents in the University of
China (NCET-10-0383) and the Chutian Scholar Program of the
Hubei provincial government are also acknowledged. The
authors also thank the Specialized Research Fund for the
Doctoral Program of Higher Education (20090142120081).
16 D. B. Ramachary, Y. V. Reddy and M. Kishor, Org. Biomol. Chem.,
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Green Chem.
This journal is © The Royal Society of Chemistry 2012